Polymer nanofiber scaffolds and uses thereof

ABSTRACT

A polymer nanofiber scaffold includes a plurality of melt extruded nanofibers that are chemically modified to append surface functionality to the nanofibers.

RELATED APPLICATION

This application claims priority from U.S. Provisional Application No. 61/937,756, filed Feb. 10, 2014, the subject matter of which is incorporated herein by reference in its entirety.

GOVERNMENT FUNDING

This invention was made with government support under Grant No. R00EB011530 awarded by The National Institutes of Health and DMR 0423914 awarded by The National Science Foundation. The United States government has certain rights to the invention.

BACKGROUND

Traditional industrial processes for synthetic polymer microfiber spinning can be classified as either solvent-based or melt-based. As the name implies, solvent processing involves the spinning of a polymer solution with solidification of the fiber either through coagulation in a non-solvent (wet-spinning) or solvent evaporation (dry-spinning) In contrast, melt spinning produces fibers via the spinning of molten polymer that solidifies upon cooling; drawing usually accompanies this melt-based process to induce chain orientation and enhance mechanical properties. Typically, accessing nanoscale cross-sections is difficult, where fiber diameters of only 10-20 μm are achieved with applications in the textile industry. Pushing the limits of fiber production to the nanoscale has garnered recent attention in the processing arena.

Electrospinning is perhaps the most well-known, and one of the oldest techniques for generating sub-micron fibers in lab-scale from a polymer solution, or less commonly, a polymer melt via the application of a large electric field. This charged polymer jet is subjected to electrostatic forces, which act to elongate, thin, and solidify the polymer fiber in the characteristic “whipping instability” region. Although some success has been achieved with electrospun fibers in high-value added applications, such as air filtration, topical drug delivery, and tissue engineering scaffolds, significant disadvantages are low throughput and scalability. Additionally, electrospinning necessitates large volumes of toxic solvents that must be recovered by specialized equipment to make the process viable on a large scale.

Other approaches to nanofiber fabrication have emerged, including rotary-jet spinning, gas jet blowing, melt blowing, and bicomponent fiber spinning Recent advances in rotary jet spinning have focused on a melt-based approach to nanofiber production with throughputs significantly higher than electrospinning, but improvements are ongoing to address processing complexity as it relates to broad applicability to a range of polymer systems. Melt blowing is a particularly commercially relevant and scalable technique for achieving fiber diameters on the order of tens of microns and higher; in this process, fibers are generated in-line by extrusion of a polymer through a die orifice, while a hot air jet blows down the extrudate. It is process compatible with a wide range of polymers, and is a solvent-less and environmentally-friendly manufacturing method. However, the pursuit of nanoscale fibers has been limited primarily to polypropylene for air filtration. Collectively, these limitations on nanofiber scalability increase manufacturing costs and lower productivity.

Polymeric materials have become ubiquitous in regenerative medicine as scaffolds for cell-seeding, where they have found application in the induction of cellular adhesion, proliferation, and differentiation. Nano-fibrous scaffolds are of particular use as they are porous, allowing transport of nutrients and waste products, have high surface area to volume ratios, and can provide directed cell growth based on fiber alignment. Synthetic fibers for regenerative medicine are usually comprised of polyesters, often poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA) or poly(caprolactone) (PCL), due to their degradability via hydrolytic pathways and resultant non-toxic byproducts. However, most polymeric scaffolds are unable to promote biological effects, as synthetic polymers do not possess the biochemical cues that are necessary to impact a cell's fate.

Modification of these polyester fibers typically relies on the degradation of the polymer chains, either through hydrolysis to expose carboxylic acids and alcohols, or through aminolysis to expose a second functional group off of the amine. Both of these routes degrade the polymer, potentially resulting in reduced mechanical properties and increased erosion of the fibers. Recent work has aimed to ameliorate this through the synthesis of telechelic polymers, which could then be processed into a scaffold.

SUMMARY

Embodiments described herein relate to a polymer nanofiber scaffolds and to their use in, for example, tissue engineering, drug delivery, wound healing, chemical processing, nanoelectronics, fabrics, and filtration applications. In some embodiments, a polymer nanofiber scaffold can include a plurality of melt extruded polymer nanofibers. The nanofibers can each have a rectangular cross-section defined in part by an encapsulating polymer material that is separated from the nanofibers. The nanofibers include a plurality of click-reactive functional groups of specific binding pairs extending from portions of outer surfaces of the nanofibers. The functional groups can be chemically bound to the nanofibers without degrading polymer chains of the nanofibers. The functional groups can be appended to complementary click-reactive groups of the specific binding pair that are conjugated to at least one agent.

In some embodiments, the concentration of functional groups extending from the at least one portion can be at least about 0.1 nmol/cm². The agent conjugated to complementary click-reactive group, which can be appended to the functional group, can include at least one of a bioactive agent, diagnostic agent, therapeutic agent, catalyst, charged molecule, peptide, polypeptide, nucleic acid, polynucleotide, small molecule, nanoparticle, antibody, carbohydrate, or vector.

In other embodiments, the functional groups can be spatially arranged on the nanofibers such that a first portion of the nanofibers has a first concentration of functional groups and a second portion of the nanofibers has a second concentration of functional groups different than the concentration of the first portion, e.g., different portions of the nanofibers can have different concentrations of the same or different functional groups. In some embodiments, the functional groups can be arranged on the nanofibers in a concentration gradient.

In still other embodiments, the plurality of click-reactive function groups can include first click reactive functional groups and second click reactive functional groups different than the first click reactive functional groups.

In some embodiments, the functional groups are chemically bound to the nanofibers with diarylhydroxymethylene linkages that are formed by reaction of click-reactive functional group substituted diarylketones with the polymer chains of the nanofibers. The click-reactive functional groups can include at least one of an amine, sulfate, thiol, hydroxyl, azide, alkyne, alkene, carboxyl groups, aldehyde groups, sulfone groups, vinylsulfone groups, isocyanate groups, acid anhydride groups, epoxide groups, aziridine groups, episulfide groups, —CO₂N(COCH₂)₂, —CO₂N(COCH₂)₂, —CO₂H, —CHO, —CHOCH₂, —N═C═O, —SO₂CH═CH₂, —N(COCH)₂, —S—S—(C₅H₄N) and groups of the following structures,

-   -   wherein R₁ is hydrogen or C₁ to C₄ alkyl.

In other embodiments, the nanofibers can be formed of a biocompatible or cytocompatible polymer, such as a polyester (e.g., polycaprolactone).

Other embodiments described herein relate to a method of forming an agent functionalized polymer nanofiber scaffold. The method includes providing a plurality of melt extruded polymer nanofibers. The nanofibers can each have a rectangular cross-section defined in part by an encapsulating polymer material that is separated from the nanofibers. A plurality of click-reactive functional groups of a specific binding pair can then be chemically bound to the fibers without degrading polymer chains of the nanofibers. The click-reactive functional groups can extend from portions of outer surfaces of the nanofibers. At least one agent is appended to the nanofibers by reacting the agent conjugated to a complementary click-reactive group of the specific binding pair with the click-reactive functional groups of the nanofibers.

In some embodiments, the agent can include at least one bioactive agent, diagnostic agent, therapeutic agent, catalyst, charged molecule, peptide, polypeptide, nucleic acid, polynucleotide, small molecule, nanoparticle, antibody, carbohydrate, or vector.

In other embodiments, the nanofibers can be formed by coextruding a first polymer material with a second polymer material to form a coextruded polymer film having discrete overlapping layers of polymeric material; multiplying the overlapping layers to form a multilayered composite film; and separating the first polymer material from the second polymer material to form the plurality of nanofibers having the rectangular cross-section. Separating the polymer materials can include, for example, subjecting the multilayered composite film to a high pressure water stream or a high pressure air stream, or dissolving the second polymer material.

In some embodiments, the functional groups are spatially arranged on the nanofibers such that a first portion of the nanofibers has a first concentration of functional groups and a second portion of the nanofibers has a second concentration of functional groups different than the concentration of first portion. The functional groups can be appended to complementary click-reactive groups that are conjugated to at least one agent to provide a first concentration of agents on the first portion and a second concentration of agents on the second portion.

In other embodiments, the functional groups can be spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the functional groups. The functional groups can be appended to complementary click-reactive groups that are conjugated to at least one agent to provide different concentrations of agents on different portions of the nanofibers.

In still other embodiments, the plurality of click-reactive function groups can include first click reactive functional groups and second click reactive functional groups different than the first click reactive functional groups. The first click reactive functional groups can appended to first agents and the second click reactive functional groups can be appended to second agents different than the first agents.

In some embodiments, the concentration of the functional click-reactive function groups and appended agent extending from at least one portion of the nanofibers is at least about 0.1 nmol/cm². The appended agent can be a bioactive agent that promotes at least one of cell adhesion, growth, or proliferation and the polymer scaffold can be used in at least one of biomedical, tissue engineering, and/or wound healing applications, such as a wound dressing or engineered tissue construct.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a coextrusion and layer multiplying process used to form a multilayered polymer composite film in accordance with an embodiment.

FIG. 2 is a schematic illustration of coextruding skin layers onto the composite film of FIG. 1 to form a composite stream.

FIG. 3 is a schematic illustration of additional layer multiplying steps for the composite stream of FIG. 2.

FIG. 4A is a schematic illustration of stretching the composite stream of FIG. 2.

FIG. 4B is a schematic illustration of compressing the composite stream of FIG. 2.

FIG. 4C is a schematic illustration of delaminating the composite stream of FIG. 2.

FIG. 5 is a schematic illustration of delaminating a composite stream to form a scaffold.

FIG. 6 is a schematic illustration of a nanofiber surface functionalized with two different chemistries

FIGS. 7(A-B) illustrate: (A) schematic of co-extrusion and 2-dimensional multiplication system for producing nanofibers (top); and (B) scanning electron micrograph of the as-extruded nanofibers following PEO dissolution (bottom). Scale bar (left)=20 μm, right=50 μm.

FIG. 8 illustrates a chemical scheme for modification of PCL nanofibers.

FIG. 9 illustrates fluorescent confocal micrographs of PCL nanofibers. A) PCL nanofiber control with no CuSO₄ added during reaction with AF₄₈₈. Scale bar=50 μm. B) PCL after the CuAAC reaction with AF₄₈₈, including CuSO₄. Scale bar=50 μm. C) Fluorescent intensity in the region of interest, as indicated by the red lines in images A and B. The red line on the graph corresponds to image A, and the black line is indicative of image B.

FIG. 10(A-F) illustrate: A) Full XPS spectrum of PCL, PCL-PrBz, and PCL-RGD. B) N_(1s) XPS spectrum of PCL nanofibers and PCL-RGD fibers, C) Confocal fluorescent microscopy image of NIH 3T3 cells after 72 hours of growth on PCL-RGD scaffold. 10 x objective. D) Confocal fluorescent microscopy image of NIH 3T3 cells after 72 hours of growth on PCL-RGD scaffold. 40× objective. E) Confocal fluorescent microscopy image of NIH 3T3 cells after 72 hours of growth on control PCL scaffold. 10 x objective. F) Confocal fluorescent microscopy image of NIH 3T3 cells after 72 hours of growth on control PCL scaffold. 40 x objective. Blue indicates DAPI stain and green indicates actin green stain in confocal micrographs.

FIG. 11 illustrates a schematic of a synthesis scheme to generate a PCL fiber gradient.

FIG. 12(A-C) illustrate: A) schematic of gradient photomask as inkjet printed onto transparency sheets: (a) is 100% black, (b) is 50% of black and (c) is transparent as indicated in the text; B) Fluorescence image of PCL-AF488 fiber gradient; and C) plot of fluorescence intensity over the total fiber distance, indicating an approximate linear gradient of intensity.

FIG. 13(A-C) illustrate: A) Digital image of PCL gradient-modified IKVAV fibers, indicating points where IR spectra were taken. B) ATR-FTIR spectrum of varying spots on the PCL gradient (indicated in A). The arrow indicates the amide I region derived from the IKVAV peptide. C) ATR-FTIR imaging at 1628 cm⁻¹ (scale bar=50 μm, intensity is indicated by a heat map as is indicated on the right).

FIG. 14(A-C) illustrate: A) Digital image of IKVAV gradient fibers, indicating spots taken for water contact angle and X-ray photoelectron spectroscopy. B) Water contact angle)(°) of PCL-g-IKVAV fibers with four distinct spots from left (low peptide concentration) to right (high peptide concentration). Error bars represent standard deviation (n=3). C) Full XPS wide scan from left to right of PCL-gradient (spots 1-4), as indicated in A. IKVAV fibers shows increasing intensity of nitrogen (N1s) with increasing UV irradiation intensity.

FIG. 15(A-I) illustrate confocal microscopy images of PC-12 cells. Cells were cultured for 5 days and stained with DAPI and anti β-III-tubulin. Fiber orientation is approximately horizontal. A-C) PC-12 cells cultured on unmodified PCL, corresponding to spots (a), (b) and (c). D-F) PC-12 cells cultured on PCL-ng-IKVAV which correspond spot (a), (b) and (c) in FIG. 12A. G-I) PC-12 cells cultured on 3 different regions of PCL-g-IKVAV which correspond spot (a), (b) and (c). The direction of the gradient is left to right, where G is the lowest peptide concentration and I is the highest. (Scale bar=50 μm).

FIG. 16(A-C) illustrate non-gradient photomasks which correspond to different intensities of the gradient photomask. A) 100% black photomask corresponding to the least amount of UV fluence in the gradient photomask. B) 50% black photomask where UV intensity corresponds to the exact center of the gradient photomask. C) Transparent photomask corresponding to the highest UV fluence through the gradient photomask.

FIG. 17 illustrates images showing AF₄₈₈ clicked PCL fiberbundles; Top: Digital image of fibers corresponding to the individual photomasks in FIG. 16. Groups A, B, and C fibers used photomask A, B, and C to perform photochemistry with Pr-Bz before click chemistry. The table indicates the intensity of UV light and the surface coverage of dyes conjugated onto fibers (n=3).

DETAILED DESCRIPTION

Embodiments described herein relate to surface functionalized polymer nano-fibrous (or nanofiber) scaffolds, methods of forming the surface functionalized polymer nanofiber scaffolds, and to the use of the scaffolds in, for example, tissue engineering, drug delivery, wound healing, chemical processing, nanoelectronics, fabrics, and filtration applications. The nanofiber scaffolds can include a plurality of non-woven or woven nanofibers that are formed from commodity polymers using a continuous extrusion process. The nanofibers can have a rectangular cross-section of about 10 nm (height)×10 nm (width) to about 10 μm×10 μm, with variations in between, and a surface area of at least about 1 cm²/g or more. The nanofibers and/or scaffold formed from the nanofibers can be chemically modified (or surface modified) non-destructively after formation to chemically bond click reactive functional groups of specific binding pairs onto the nanofibers such that the click-reactive functional groups extend uniformly across the surface of the nanofibers and/or scaffold or from selected portions of the nanofibers or scaffold at varying concentrations, types, and/or densities. Complementary click-reactive groups of the specific binding pairs conjugated to any number of any number of agents and/or chemical entities can be click-reacted with the click-reactive groups chemically bound to the nanofibers to the agents to the nanofibers. The agents appended to the click reactive groups of the fibers can include, for example, bioactive agents, diagnostic agents, therapeutic agents, catalysts, charged molecules, peptides, polypeptides, nucleic acids, polynucleotides, small molecules, nanoparticles, antibodys, carbohydrates, and vectors.

In some embodiments, one or more similar or different functional groups can be spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the functional groups. For example, a plurality of first functional groups can be arranged on the nanofibers in a first concentration gradient and/or a plurality of second functional groups can be arranged on the nanofibers in a second concentration gradient.

Advantageously, the nanofiber scaffold can be formed or fabricated using solely commercially available polymers, such as PCL and poly (ethylene oxide) (PEO). The process used to form the nanofiber scaffold can be solvent-free, allow for controllable cross-sectional dimensions of the fibers, and use FDA-friendly polymers during processing. The fabrication process is flexible because it involves the use of an extrusion line that is composed of several basic multipliers. Arrangement of these multipliers allows control over the number, as well as the dimensions, of fibers contained in one extrudate. In addition, the cross-sectional geometry of the nanofibers is rectangular creating greater surface area to volume ratios (e.g., at least about 1 cm²/mg, at least about 10 cm²/mg, at least about 20 cm²/mg, at least about 40 cm²/mg, at least about 50 cm²/mg or more), when compared to cylindrical fibers. The increased surface area can allow for a higher concentration of surface modifications to be available on the fiber, potentially improving the display of chemical or biochemical cues.

The surface functionalized polymer nanofiber scaffold can be formed from a multilayered polymer composite film. FIG. 1 illustrates a coextrusion and multilayering process used to form a multilayered polymer composite film 10. First, a first polymer layer 12 and a second polymer layer 14 are provided. The first layer 12 is formed from a first polymeric material (a) and the second polymer layer 14 is formed from a second polymer material (b) that is substantially immiscible and has a similar viscosity with the first polymer material (a) when coextruded. It will be appreciated that one or more additional layers formed from the polymer materials (a) or (b) or a different polymer materials may be provided to produce the multilayered polymer composite film 10.

The term “polymer” or “polymeric material” as used in the present application denotes a material having a weight average molecular weight (Mw) of at least 5,000. Preferably the polymer is an organic polymeric material. The term “oligomer” or “oligomeric material” as used in the present application denotes a material with a weight average molecular weight of from 1,000 to less than 5,000. Such polymeric materials can be glassy, crystalline or elastomeric polymeric materials.

Examples of polymeric materials that can potentially be used for the first and second polymer materials (a), (b) include, but are not limited to, polyesters such as polylactic acid, poly(lactic-co-glycolic acid), polycaprolactone (PCL); and poly(ethylene naphthalate) and isomers thereof, such as 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-polyethylene naphthalate; polyalkylene terephthalates such as polyethylene terephthalate, polybutylene terephthalate, and poly-1,4-cyclohexanedimethylene terephthalate; polyimides, such as polyacrylic imides; polyetherimides; styrenic polymers, such as atactic, isotactic and syndiotactic polystyrene, α-methyl-polystyrene, para-methyl-polystyrene; polycarbonates, such as bisphenol-A-polycarbonate (PC); polyethylenes, such as polyethyele oxide (PEO); poly(meth)acrylates, such as poly(isobutyl methacrylate), poly(propyl methacrylate), poly(ethyl methacrylate), poly(methyl methacrylate), poly(butyl acrylate) and poly(methyl acrylate) (the term “(meth)acrylate” is used herein to denote acrylate or methacrylate); cellulose derivatives, such as ethyl cellulose, cellulose acetate, cellulose propionate, cellulose acetate butyrate, and cellulose nitrate; polyalkylene polymers, such as polyethylene, polypropylene, polybutylene, polyisobutylene, and poly(4-methyl)pentene; fluorinated polymers such as perfluoroalkoxy resins, polytetrafluoroethylene, fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene and copolymers thereof; chlorinated polymers, such as polydichlorostyrene, polyvinylidene chloride and polyvinylchloride; polysulfones; polyethersulfones; polyacrylonitrile; polyamides such as nylon, nylon 6,6, polycaprolactam, and polyamide 6 (PA6); polyvinylacetate; and polyether-amides. Also suitable are copolymers, such as styrene-acrylonitrile copolymer (SAN), preferably containing between 10 and 50 wt %, preferably between 20 and 40 wt %, acrylonitrile, styrene-ethylene copolymer; and poly(ethylene-1,4-cyclohex-ylenedimethylene terephthalate) (PETG). Additional polymeric materials include an acrylic rubber; isoprene (IR); isobutylene-isoprene (IIR); butadiene rubber (BR); butadiene-styrene-vinyl pyridine (PSBR); butyl rubber; chloroprene (CR); epichlorohydrin rubber; ethylene-propylene (EPM); ethylene-propylene-diene (EPDM); nitrile-butadiene (NBR); polyisoprene; silicon rubber; styrene-butadiene (SBR); and urethane rubber. Additional polymeric materials include block or graft copolymers. In one instance, the polymeric materials used to form the layers 12, 14 may constitute substantially immiscible thermoplastics. In addition, each individual layer 12, 14 may include blends of two or more of the above-described polymers or copolymers, preferably the components of the blend are substantially miscible with one another yet still maintaining substantial immiscibility between the layers 12, 14. The components comprising the layers 12, 14 can include organic or inorganic materials, including nanoparticulate materials, designed, for example, to modify the mechanical properties of the components, e.g., tensile strength. It will be appreciated that potentially any extrudable polymer can be used as the first polymer material (a) and the second polymer material (b) so long as upon coextrusion such polymer materials (a), (b) are substantially immiscible and form discrete layers or polymer regions. Such materials can have a substantially similar viscosity upon coextrusion.

In some embodiments, the polymers used to for the first and/or second polymer can be biodegradable and/or substantially biocompatible or cytocompatible (i.e., substantially non-cytotoxic). The use of biodegradable and substantially biocompatible or cytocompatible polymers allows a surface functionalized nanofiber scaffold to be formed that can be used in medical applications, e.g., tissue engineering or wound healing. Examples of that polymers that are substantially biocompatible or cytocompatible include polyesters, such as PCL, paired with PEO.

Referring to FIG. 1, the layers 12, 14 are co-extruded and multiplied in order to form the multilayered polymer composite film 10. In particular, a pair of dies 40, 50 is used to coextrude and multiply the layers 12, 14. Each layer 12, 14 initially extends in the y-direction of an x-y-z coordinate system. The y-direction defines the length of the layers 12, 14 and extends in the general direction of flow of material through the dies 40, 50. The x-direction extends transverse, e.g., perpendicular, to the y-direction and defines the width of the layers 12, 14. The z-direction extends transverse, e.g., perpendicular, to both the x-direction and the y-direction and defines the height or thickness of the layers 12, 14.

The layers 12, 14 are initially stacked in the z-direction and define an interface 20 therebetween that resides in the x-y plane. As the layers 12, 14 approach the first die 40 they are separated from one another along the z-axis to define a space 22 therebetween. The layers 12, 14 are then re-oriented as they pass through the first die 40. More specifically, the first die 40 varies the aspect ratio of each layer 12, 14 such that the layers 12, 14 extend longitudinally in the z-direction. The layers 12, 14 are also brought closer to one another until they engage or abut one another along an interface 24 that resides in the y-z plane.

The layers 12, 14 then enter the second die 50 where layer multiplication occurs. The second die 50 may constitute a single die or several dies which process the layers 12, 14 in succession (not shown). Each layer 12, 14 is multiplied in the second die 50 to produce a plurality of first layers 12 and a plurality of second layers 14 that alternate with one another to form the multilayered polymer composite film 10. Each pair of layers 12, 14 includes the interface 24 that resides in the y-z plane. The layers 12, 14 are connected to one another generally along the x-axis to form a series of discrete, alternating layers 12, 14 of polymer material (a), (b). Although three of each layer 12 and 14 are illustrated it will be appreciated that the multilayered polymer composite film 10 may include, for example, up to thousands of each layer 12, 14.

Referring to FIG. 2, once the multilayered polymer composite film 10 is formed a detachable skin or surface layer 30 is applied to the top and bottom of the film 10 such that the film 10. In particular, the multilayered polymer composite film 10 enters a die 60 where the film 10 is sandwiched between two skin layers 30 along the z-axis to form a first composite stream 100. The skin layer 30 may be formed from the first polymer material (a), the second polymer material (b) or a third polymer material (c) different from the first and second polymer materials (a), (b). One or both of the skin layers 30 may, however, be omitted (not shown).

Referring to FIG. 3, the first composite stream 100 is divided along the x-axis into a plurality of branch streams 100 a, 100 b and processed through a pair of multiplying dies 70, 80. In the die 70, the streams 100 a, 100 b are stacked in the z-direction, stretched in both the x-direction and the y-direction, and recombined to form a second composite stream 110 that includes a plurality of multilayered films 10 alternating with skin layers 30. Biaxial stretching of the branch streams 100 a, 100 b in the x-direction and y-direction may be symmetric or asymmetric.

The die 80 performs similar modifications to the second composite stream 110 that the die 70 performed on the branch streams 100 a, 100 b. In particular, in the die 80 the second composite stream 110 is divided along the x-axis, stacked along the z-axis, stretched in both the x-direction and the y-direction, and stacked in the z-direction to form a third composite stream 120. The third composite stream 120 shown in FIG. 3 includes four multilayered composite films 10 that alternate with five skin layers 30, although more or fewer of the films 10 and/or layers 30 may be present in the third composite stream 120. Regardless, the third composite stream 120 includes a plurality of layer interfaces 24 between the layers 12, 14.

By changing the volumetric flow rate of the polymer layers 12, 14 through the dies 70, 80, the thickness of both the polymer layers 12, 14 and each multilayered polymer film 10 in the z-direction can be precisely controlled. Additionally, by using detachable skin layers 30 and multiplying the composite streams 100, 110 within the dies 70, 80, the number and dimensions of the layers 12, 14, the multilayered polymer film 10, and the branch streams 100 a, 100 b in the x, y, and z-directions can be controlled.

Referring to FIGS. 4A and 4B, the first composite structure 100 may be mechanically processed by, for example, at least one of orientation (FIG. 4A), compression (FIG. 4B), and ball-mill grinding (not shown). As shown, the composite stream 100 is stretched in the y-direction as indicated generally by the arrow “S”, although the composite stream 100 may alternatively be stretched in the x-direction (not shown). FIG. 4B illustrates the composite stream 100 being compressed in the z-direction as indicated generally by the arrow “C”. The degree of stretching and/or compression will depend on the application in which the nanofibers and/or scaffold formed from the multilayered polymer film 10 is to be used. The ratio of y-directional stretching to z-direction compression may be inversely proportional or disproportional.

In one embodiment, the multilayer film can be uniaxially stretched in the y-direction or S-direction at a draw ratio of about 1 to about 10 to decrease the cross-sectional dimension of the fibers and increase the surface area of the nanofibers and scaffold so formed.

Referring to FIG. 4C, the first composite stream 100 can be further processed to cause the components 12, 14, 30 thereof to separate or delaminate from one another and form a plurality of fibers or fiber-like structures 12 a, 14 a from the layers 12, 14. The removed skin layers 30 are discarded. In one instance, the layers 12, 14, 30 are mechanically separated by high pressure water jets (not shown). In particular, two opposing ends of the composite stream 100 can be fixed and water jets with a nozzle pressure of no less than about 2000 psi can be applied to the composite stream 100 to separate the layers 12, 14, 30 completely, thereby forming the nano-fibers 12 a, 14 a. More specifically, applying high pressure water to the first composite stream 100 removes the interfaces 24 between the layers 12, 14, i.e., delaminates the multilayered polymer composite films 10, to form the fibers 12 a and 14 a. Although delamination of the first composite stream 100 is illustrated, it will be appreciated that the multilayered polymer composite film 10, the second composite stream 110 or the third composite stream 120 may likewise be delaminated via high pressure water or the like to form the fibers 12 a, 14 a.

Alternatively, the polymer material (a) or (b) of one of the layers 12, 14 is selected to be soluble in a particular solvent while the other polymer material (a) or (b) is selected to be insoluble in that solvent. Accordingly, immersing the composite stream 100 in the solvent separates the layers 12, 14 by wholly removing, e.g., dissolving, not only the interfaces 24 between the layers 12, 14 but removed the soluble layers 12 or 14 entirely. The insoluble layers 12 or 14 are therefore left behind following solvent immersion. The same solvent or a different solvent may be used to dissolve the skin layers 30, when present. The remaining soluble layers 12 or 14 form the fibers 12 a or 14 a. In one instance, the solvent can be water and in some instances no organic solvent is used.

Optionally, as illustrated in FIG. 5, a multilayer composite stream that includes a polymer material soluble in a particular solvent and another polymer material insoluble in that solvent can be bundled or consolidated prior to solvent immersion Immersing the bundled composite stream in the solvent separates the layers and forms a scaffold of the polymer nanofibers.

Whether the fibers 12 a and/or 14 a are formed by mechanically separating the layers 12 or 14 or dissolving one of the layers 12 or 14 with a solvent, the nanofibers 12 a and/or 14 a produced by the described coextrusion process have rectangular cross-sections rather than the conventional, round cross-sections formed by electrospinning. These rectangular or ribbon-like nanofibers 12 a or 14 a have a larger surface area-to-volume ratio than round fibers developed using spinning methods. Regardless of the method of separation enlisted, the nanofibers 12 a and/or 14 a can stretch, oscillate, and separate from each other at the interface 24. Furthermore, due to the aforementioned mechanical processing techniques of FIGS. 4A and 4B, the exact cross-sectional dimensions of the rectangular fibers 12 a and/or 14 a can be precisely controlled. For example, the rectangular fibers 12 a and/or 14 a can be made smaller and strengthened via mechanical processing.

Due to the construction of the first composite stream 100 and the fixed sizes of the dies 40-80, the composition of the vertical layers 12, 14 and surface layers 30 is proportional to the ratio of the height in the z-direction of a vertical layer 12, 14 section to that of a surface layer 30 section. Therefore, if the layer 12 (or 14) is selected to form the rectangular fibers 12 a (or 14 a), the thickness and height of the final fibers 12 a (or 14 a) can be adjusted by changing the ratio of the amount of the layers 12, 14 as well as the amount of surface layer 30. For example, increasing the percentage of the amount of the material (b) of the layers 14 relative to the amount of the material (a) of the layers 12 and/or increasing the amount of the material of the surface layers 30 results in smaller rectangular fibers 12 a. Alternatively, one or more of the dies 40-80 may be altered to produce nanofibers 12, 12 a, 14, 14 a having a size and rectangular cross-section commensurate with the desired application. In one instance, one or more of the dies 40-80 could be modified to have a slit or square die construction to embed the fibers 12, 12 a, 14, 14 a within the surface layers 30.

The extrusion process can be tailored to produce vertically layered films 10 with designer layer/fiber thickness distributions. For example, the relative material compositions of the polymers (a), (b) of the layers 12, 14 can be varied with great flexibility to produce rectangular polymer fibers 12, 12 a, 14, 14 a with highly variable constructions, e.g., 50/50, 30/80, 80/30, etc. The rectangular polymer fibers 12, 12 a, 14, 14 a can be highly oriented and strengthened by post-extrusion orienting. Furthermore, a wide magnitude of layer 12, 14 thicknesses in the z-direction is achievable from a few microns down to tens of nanometers depending on the particular application.

The polymer nanofibers so formed can be consolidated to produce polymer nanofiber scaffolds. In one embodiment, as shown in FIG. 5, the nanofibers can be consolidated by bundling the composite stream prior to separation of the fibers or salvation of a polymer. In other embodiments, the nanofibers can be consolidated by compressing, weaving, and physically mixing the fibers.

The nanofiber scaffolds can have various densities and porosities depending on the fiber cross-section and the process used to consolidate the fibers. For example, the scaffolds can have a porosity of about 1% by volume to about 50% by volume, and a pore size of about 1 μm to about 100 μm. In some embodiments, each fiber can have a rectangular cross section of 10 nm (height)×10 nm (width) to about 10 μm×10 μm, with variations in between. The nanofibers can have surface area of at least about 1 cm²/mg, at least about 10 cm²/mg, at least about 20 cm²/mg, at least about 40 cm²/mg, at least about 50 cm²/mg or more.

The nanofibers of the scaffold can be chemically modified (or surface modified) non-destructively after processing to append click reactive functional groups onto the nanofibers such that the click-reactive functional groups extend from portions or selected portions of the nanofiber.

In some embodiments, the click-reactive functions groups can be chemically bound to the nanofibers with diarylhydroxymethylene linkages that are formed by reaction of click-reactive functional group substituted diarylketones with polymer chains of the nanofibers. For example, modified diarylketones, such as benzophenones, which present click-reactive functional groups, such as alkynes, ketones, and alkoxyamines, can be attached to polymer chains of the nanofibers using photochemistry. In the presence of ultra-violet (UV) light, the diarylketones can generate radicals and undergo free radical insertion in the polymer backbone of the nanofibers.

For example, as shown below, photochemistry can used to chemically bind a click-reactive group substituted benzophone to PCL of a nanofiber.

-   -   wherein R is a click-reactive group.

By click-reactive functional group, it is meant that the functional group has click reactivity and/or can undergo a click reaction with a complementary click reactive functional group or molecule. Examples of the types of reactions that are known to have click reactivity include cycloaddition reactions. These reactions represent highly specific reactant pairs that have a chemoselective nature, meaning that they mainly react with each other and not other functional groups. One example of a cycloaddition reaction is the Huisgen 1,3-dipolar cycloaddition of a dipolarophile with a 1,3 dipolar component that produce five membered (hetero)cycles. Examples of dipolarophiles are alkenes, alkynes, and molecules that possess related heteroatom functional groups, such as carbonyls and nitriles. Specifically, another example is the 2+3 cycloaddition of alkyl azides and acetylenes. Other cycloaddition reactions include Diels-Alder reactions of a conjugated diene and a dienophile (such as an alkyne or alkene).

Other examples of the types of reactions that are known to have click reactivity include a hydrosilation reaction of H—Si and simple non-activated vinyl compounds, urethane formation from alcohols and isocyanates, Menshutkin reactions of tertiary amines with alkyl iodides or alkyl trifluoromethanesulfonates, Michael additions, e.g., the very efficient maleimide-thiol reaction, atom transfer radical addition reactions between —SO₂Cl and an olefin, metathesis, Staudinger reaction of phosphines with alkyl azides, oxidative coupling of thiols, many of the procedures already used in dendrimer synthesis, especially in a convergent approach, which require high selectivity and rates, nucleophilic substitution, especially of small strained rings like epoxy and aziridine compounds, carbonyl chemistry like formation of ureas, and addition reactions to carbon-carbon double bonds like dihydroxylation. Therefore, the attached click-reactive functional group may be chosen, for example, from an alkyne group, acetylene group, an azido-group, a nitrile group, acetylenic group, amino group, or phosphino group. The click chemistry reaction may results in the addition of a functional group selected from amino, primary amino, hydroxyl, sulfonate, benzotriazole, bromide, chloride, chloroformate, trimethylsilane, phosphonium bromide or bio-responsive functional group.

In some embodiments, the click-reactive group can include at least one of an amine, sulfate, thiol, hydroxyl, azide, alkyne, alkene, carboxyl groups, aldehyde groups, sulfone groups, vinylsulfone groups, isocyanate groups, acid anhydride groups, epoxide groups, aziridine groups, episulfide groups, —CO₂N(COCH₂)₂, —CO₂N(COCH₂)₂, —CO₂H, —CHO, —CHOCH₂, —N═C═O, —SO₂CH═CH₂, —N(COCH)₂, —S—S—(C₅H₄N) and groups of the following structures,

-   -   wherein R₁ is hydrogen or C₁ to C₄ alkyl.

The click-reactive functional group substituted diarylketones can be provided on the nanofibers or nanofiber scaffold by coating a solution of the click-reactive functional group substituted diarylketones onto the nanofibers, removing excess solvent, and irradiating the coated nanofibers or scaffold with UV light. In some embodiments, the solution of click-reactive functional group substituted diarylketones can be coated, e.g., spray coated, on the nanofibers of the scaffold such that the concentration of functional groups extending from at least one portion is at least about 0.1 nmol/cm². The amount, concentration, and/or spatial location of the click-reactive functional group substituted diarylketones provided on the nanofibers of the scaffold can be controlled such that different densities or concentrations of the click-reactive functional group substituted diarylketones are localized or patterned on different portions of the scaffold. In some embodiments, surface coverage of the nanofibers can be, for example, about 0.1 nmol/cm² to about 10 nmol/cm².

In some embodiments, one or more similar or different click-reactive functional groups can be spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the same or different click-reactive functional groups. For example, a plurality of first functional groups can be arranged on the nanofibers in a first concentration gradient and/or a plurality of second functional groups can be arranged on the nanofibers in a second concentration gradient.

In some embodiments, gradient concentrations of the click-reactive functional groups can be provided on the fibers or scaffold by spraying or coating different portions of the nanofibers and/or scaffold different with different types or concentrations of click-reactive functional group substituted diarylketones. In other embodiments, gradient concentrations of the click-reactive functional groups can be provided on the fibers or scaffold using a photomasking strategy to pattern or modulate the concentration of click-reactive functional group substituted diarylketones deposited on select portions of the fibers. For example, photomask having defined regions of transparency ranging from completely UV transparent to opaque can be used to pattern or define the surface location and concentration of the click-reactive functional group substituted diarylketones bound to the nanofibers of the scaffold.

The gradients can be comprised of different click-reactive functional group substituted diarylketones bound to the nanofibers of the scaffold, such as benzophenones having different click-reactive functional groups (e.g., alkyne, aminoxy, etc.). As discussed below, for example, the different click-reactive functional group substituted diarylketones bound to the nanofibers can be provided in a particular pattern or concentration on the nanofiber scaffold to allow the attachment of different agents to the nanofiber scaffold.

For example, as shown in FIG. 6, a first solution of ketone click-reactive group substituted diarylketones can be provided on a PCL nanofiber and irradiated with UV light through a photomask with defined transparency regions to provide the PCL nanofiber with a defined concentration gradient of the ketone click-reactive group substituted diarylketones. A second solution of alkyne click-reactive group substituted diarylketones can then be provided on the PCL nanofiber and irradiated with UV light through a photomask with defined transparency regions to provide the PCL nanofiber with a defined concentration gradient of the alkyne click-reactive group substituted diarylketones.

The click-reactive functional groups bound to the polymer chains of the nanofibers using diarylketone chemistry can participate in click-reactions, such as cycloaddition or Michael addition reactions, with complementary click-reactive groups of specific binding pairs. The complementary click-reactive groups of specific binding pairs can be conjugated to one more agents such that the agents can be readily attached to the surface functionalized nanofiber using click-reactive chemistry. Examples of click-reactions used to conjugate agents include the following:

-   -   wherein R₂ is one or more agent that can be conjugated to the         surface functionalized nanofiber.

Binding different click-reactive group substituted diarylketones to the nanofiber can allow the attachment of different agents conjugated to different complementary click-reactive groups. This allows one or more similar or different agents to be spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the click-reacted agent. For example, a plurality of first agents can be arranged on the nanofibers in a first concentration gradient and/or a plurality of agents can be arranged on the nanofibers in a second concentration gradient depending on the concentration gradients of the bound click-reactive functional groups.

The agents attached to the surface functionalized nanofibers and/or scaffold using click-reactive chemistry can include any compound, small molecule, peptide, polypeptide, carbohydrate, nucleic acid, bioactive agent, diagnostic agent, therapeutic agent, catalyst, charged molecule, nanoparticle, antibody, carbohydrate, cell, semiconductor, or vector or any other material that can modify at least one physical, chemical, or functional property of the nanofibers or scaffold. The specific agent bound to the surface functionalized nanofiber can depend on the specific application, use, or function of the nanofibers and/or nanofiber scaffold.

In some embodiments, the surface functionalized nano-fiber scaffolds can be used in a variety of biomedical applications, including tissue engineering, drug delivery, wound healing, and regenerative medicine applications. Advantageous, such scaffolds can have high surface area (e.g., greater than about 10 cm²/mg of fiber) and high aspect ratio fibers for promoting cell adherence, growth, proliferation, and differentiation as well as facilitate liquid and gas transport (e.g., facilitate oxygenation of tissue and efflux of waste products from the tissue). The scaffolds can also include one or more bioactive agents and/or therapeutic agents that provide improved and/or directed cell adherence, growth, proliferation, and differentiation. Bioactive agents that can be click-reacted to the surface fuctionalized nanofiber scaffold can include any bioactive agent capable of promoting cell adhesion, growth, proliferation, and differentiation, tissue formation, destruction, and/or targeting a specific disease state. Examples of bioactive agents include chemotactic agents, various proteins (e.g., short term peptides, bone morphogenic proteins, collagen, glycoproteins, and lipoprotein), cell attachment mediators, biologically active ligands, integrin binding sequence, various growth and/or differentiation agents and fragments thereof (e.g., EGF), HGF, VEGF, fibroblast growth factors (e.g., bFGF), PDGF, insulin-like growth factor (e.g., IGF-I, IGF-II) and transforming growth factors (e.g., TGF-βI-III), parathyroid hormone, parathyroid hormone related peptide, bone morphogenic proteins (e.g., BMP-2, BMP-4, BMP-6, BMP-7, BMP-12, BMP-13, BMP-14), sonic hedgehog, growth differentiation factors (e.g., GDF5, GDF6, GDF8), recombinant human growth factors (e.g., MP-52 and the MP-52 variant rhGDF-5), cartilage-derived morphogenic proteins (CDMP-1, CDMP-2, CDMP-3), small molecules that affect the upregulation of specific growth factors, tenascin-C, hyaluronic acid, chondroitin sulfate, fibronectin, decorin, thromboelastin, thrombin-derived peptides, heparin-binding domains, heparin, heparin sulfate, polynucleotides, DNA fragments, DNA plasmids, MMPs, TIMPs, interfering RNA molecules, such as siRNAs, DNA encoding for an shRNA of interest, oligonucleotides, proteoglycans, glycoproteins, and glycosaminoglycans.

In one example, the surface functionalized nanofiber scaffold can be used as a substrate for a wound dressing. The wound dressing can have a length, a width, and a thickness that may be varied depending upon the particular application. For example, the wound healing dressing can have a sleeve-like or cylindrical configuration as well as a rectangular or square-shaped configuration. In one example, the wound healing dressing can have a thickness of about 1 cm to about 10 cm or greater, a length of about 2 cm to about 30 cm or greater, and a width of about 2 cm to about 30 cm or greater.

The wound dressing can include a plurality of peptides and proteins click-reacted to the surface of the scaffold and that can promote cell growth, attachment, and migration. An example of a protein that can be click-reacted to the surface functionalized nanofiber scaffold is epidermal growth factor (EGF). EGF is known to enhance treatment of chronic wounds. Examples of peptides that can be click-reacted to the surface functionalized nanofiber scaffold include an RGD sequence derived from fibronectin, which is known to promote cellular adhesion, and an IKVAV peptide, which can be used to induce neural growth and migration.

The EGF, RGD, and/or IKVAV can be provided or patterned in multiple gradients on the nanofiber scaffold to promote cell adherence, growth, migration. The gradients can run in the same or opposite directions. For example, the wound healing dressing formed from the surface functionalized nanofiber scaffold can be can include gradients of the EGG, RGD, and/or IKVAV that facilitate cell growth, attachment, migration when the wound healing dressing is applied to tissue being treated in a subject in need thereof. In some embodiments a gradient of IKVAV can be provided on the nanofibers and/or scaffold to promote neural cell growth and migration.

It will be appreciated that the surface functionalized nanofibers and/or scaffold can be potentially used in any application where it is desirable to modify at least one physical, chemical, or functional property of the nanofibers and/or scaffold. For example, the surface functionalized nanofibers can be used to form surface functionalized membrane supports in which various chemicals, catalysts, or other agents are patterned on the surface of the membrane. Such membranes can be useful in different processes, such as filtration, desalination, water purification, chemical processing or synthesis, nanoelectronics, non-woven fabrics, drug delivery, drug discovery, information storage, mechanical actuation, microarrays, and bioconjugation.

The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

Example 1

In this example, we used a melt co-extrusion in conjunction with a modular chemistry to yield polyester nanofibers with pendant surface functionality. The processing method makes use of the co-extrusion of PCL and PEO through a series of dye multipliers to form a nanofibrous PCL matrix within a PEO tape. PCL and PEO are melt-pumped and layered on top of one another in the extrusion line (FIG. 7). From here a vertical multiplier is used to rearrange the extrudate to yield a vertical layer structure (Step A). This is then followed by a series of vertical multipliers that cut the flow horizontally and expands the flow fields in the vertical direction to align in parallel. This process is repeated eight times to yield a vertically aligned, layered flow comprised of alternating PCL (512 layers) and PEO (512 layers) with 1024 total layers (Step B). The tape is then combined with two surface layers of PEO on the top and bottom (Step C) and is split vertically with one side stacked on top of the other (2×) to yield a PCL nanomatrix embedded in a PEO tape. This procedure is repeated one additional time, and an extrudate containing 512 PCL nanodomains embedded in a PEO matrix is obtained (Step D). The PEO in the tape can be removed by dissolution in a water bath or by using a high pressure water jet to produce a PCL nanofiber matrix. A scanning electron micrograph of the PCL fibers after the dissolving procedure shows fibers displaying cross-sectional dimensions of approximately 400-1000 nm by 2-5 μm (FIG. 7). After removal of the PEO matrix, <1% of PEO remained as determined by NMR and DSC (Supporting Information), yielding a scalable PCL nanofiber mat.

Once the fibers were extruded and separated, we sought to further modify the fiber mats to add pendant functionality. A number of papers have focused on the aminolysis of polyester fibers, in which a bifunctional amine is used to introduce a reactive group for further derivatization. In the case of the extruded nanofibers, prepared as described, the aminolysis reaction using either propargylamine or hexamethylene diamine showed minimal surface modification. This could be due to the small fraction of PEO remaining on the exterior of the fibers, occluding access to the aminolysis pathway. Rather than pursuing forcing reaction condition, an alternate route was chosen so as not to degrade the polymer chains within the fibers. As such, a photochemical radical reaction was employed to perform a C—H bond insertion into the backbone of the PCL polymer chains (FIG. 8). Benzophenone is commonly used to perform photochemical C—H insertions and has been previously used to modify PCL surfaces. Propargyl benzophenone was prepared in high-yield by reacting 4-hydroxyl benzophenone with propargyl bromide under basic conditions. The propargylbenzophenone derivative was solvent cast onto the PCL fibers, irradiated via UV (33.2 mW/cm², 30 min on each side, 320-500 nm) and washed with methanol to yield alkyne-decorated PCL nanofibers. The propargyl group could then be used in the CuAAC reaction to decorate the fiber with any azide containing molecule.

To probe the CuAAC chemistry, we first modified the PCL-alkyne fibers with an azide containing fluorescent dye, AzideFluor488 (AF₄₈₈). The fibers were modified using aqueous conditions optimized for the ligand-accelerated CuAAC reaction, using tris(3-hydroxypropyltriazolylmethyl)amine (THPTA) as the ligand to accelerate the reaction. After the CuAAC reaction, upon visual inspection, the fibers were noticeably red after using standard reaction conditions. While in the absence of copper, there was no visible change that could be attributed to a hydrophobic physical adsorption of the dye onto the fiber. To further probe the surface coverage of the PCL fibers, confocal fluorescent microscopy was used to qualitatively and quantitatively evaluate the surface coverage of the fibers. As seen in the case with no copper catalyst, negligible fluorescence is visible in the micrograph (FIG. 9A), indicating that little dye non-specifically adsorbs to the fiber bundle. However, in the presence of copper a significant fluorescent intensity appears in the micrograph indicating that the chemistry is specific, rather than simple adsorption. When fluorescence intensity was quantified, modified fibers showed approximately two-orders of magnitude higher fluorescent intensity relative to control fibers (FIG. 9C).

In order to effect biological outcomes, it is necessary that a significant surface coverage of the fibers be attained. To quantify this, we determined both the total dye loading of the fibers in conjunction with surface area measurements. Brunauer Emmett Teller (BET) measurements were used to determine the surface area of the extruded fibers, and UV-visible measurements were used to quantify dye loading. The BET technique relies on the surface adsorption of gas molecules; the total mass of the gas adsorbed allows one to calculate the area of the surface. BET measurements were carried out, showing the surface are of the fibers to be 41.2 cm²/mg of fiber. Of note, electrospun fibers of similar cross sectional dimensions showed an approximately 20-fold lower surface area. This can be attributed to the rectangular geometry of the extruded fiber as compared to the electrospun cylindrical fiber. Once the surface area was determined, we labeled the fibers with AF₄₈₈, as described. The fibers were dissolved in DCM and the absorbance at 501 nm was compared to a standard curve of AF₄₈₈ to determine total surface loading of the fibers. The PCL-AF₄₈₈ fibers were decorated with approximately 15.5 nmol/mg of fiber, resulting in a surface coverage of 0.38 nmol/cm². The surface coverage of the fibers is of particular importance as a scaffold for regenerative medicine. A common peptide, and one we employed here, is the RGD motif. RGD sequences bind to integrin receptors, and when immobilized on a surface, can lead to cell adhesion and spreading, a key feature for tissue engineering scaffolds. Previous work showed that low pM/cm² surface coverages of RGD peptides on polymeric surfaces were sufficient to promote cell adhesion. Our concentrations are three-orders of magnitude higher than those necessary for adhesion, representing a viable scaffold to present biochemical cues and for cell-seeding.

Finally, we sought to adhere an RGD peptide to promote cellular adhesion onto the PCL fiber scaffold. We first synthesized an RGD peptide with an N-terminal azide group (N₃-RGD), for attachment to the propargyl PCL fiber. The RGD peptide motif was chosen for its ability to promote adhesion through the interaction with integrin cell surface receptors, as is known to occur with NIH 3T3 fibroblasts, a common model for cell-seeding in regenerative medicine. The same scheme of photochemical attachment of propargyl-benzophenone followed by CuAAC reaction was followed to introduce the RGD-azide onto the surface of the PCL fibers. The attachment of the azido-peptide was confirmed via X-ray photoelectron spectroscopy (XPS), where the modified fiber showed a significant nitrogen peak (NO as would be expected for peptide modified fibers, while no nitrogen peak was observed upon analysis of the PCL or PCL-benzophenone (FIG. 10B). XPS surface analysis indicated an approximate surface coverage of 2% by mass, consistent with our results using the fluorescent azide.

PCL-RGD nanofibers were used as a cell seeding scaffold that would be able to promote adhesion, growth, and proliferation of NIH 3T3 fibroblasts. Both PCL and PCL-RGD fibers were immobilized on a glass slide and NIH 3T3 cells were deposited onto the fiber. Following 72 hours of incubation, cells were fixed, permeabilized, and stained using actin green and DAPI. The slides were visualized via fluorescent confocal microscopy (FIG. 10). After 72 hours, a much greater portion of the cells adhered to the fibers, as visualized by confocal microscopy. Additionally, the RGD-immobilized fibers provided enhanced cell spreading, as visualized by the actin filaments within the cells. In FIGS. 10C and 9D, at 72 hours post-seeding, the cells have become elongated along the axis of the fiber and their proliferation was enhanced by visual inspection of confocal micrographs. The unmodified fibers showed very little adhesion and proliferation after 72 hours. To quantify proliferation of the cells on the fiber scaffold, a cell viability assay was used (i.e., MTT). After 72 hours of incubation, the fibers were removed from the slides and immersed in MTT containing media. The PCL-RGD fibers showed an approximate increase of 60%, relative to the PCL fibers alone after 72 hours of incubation. These results indicate that the PCL-RGD fibers maintained the biological activity of the peptide in sufficient concentrations to promote adhesion, elongation, and proliferation.

This example shows the chemical modification of a continuously processed nanofibrous biomaterial comprised solely from commodity polymers. The processing technique is solvent free, scalable, uses FDA-friendly polymers and allows for the simple tuning of cross-sectional dimensions of the fiber. To convert this scaffold into a biological material, we utilized photochemistry to introduce an alkyne on to the surface of the fibers. This functional group allows for the modular synthesis of a host of chemically derivatized fibers by employing the CuAAC reaction. A densely covered surface was obtained using this technique, and more importantly the biological activity of the RGD peptide remained intact promoting cellular adhesion and spreading.

Example 2

In this Example, we show the incorporation of a gradient of covalently immobilized IKVAV peptides on coextruded PCL fibers to induce neural differentiation and alignment. Coextruded fibers were chosen due to their extensional strength, while maintaining lateral flexibility, in an effort to mimic a spinal cord replacement. Additionally, extruded fibers can remain aligned, providing for an additional topographical cue to promote directed elongation of neural cells. Photochemistry is used as a versatile tool to manipulate surface gradients, using a simple photomask with aligned PCL fibers. After attaching IKVAV gradients, we investigate neural cell growth in response to the gradient. In our system, both gradient modification and fiber alignment dictate neural differentiation and cellular alignment.

Materials

N, N-dimethylformamide (DMF) (99%), dimethyl sulfoxide (DMSO), methanol (99.8%), Azide-fluor 488 (HPLC), 5-bromovaleric acid, sodium azide, trifluoroacetic acid (TFA) were purchased from Sigma Aldrich. Propargyl benzophenone was prepared as previously reported. Tris(3-hydroxypropyltriazolylmethyl)amine (THPTA) was a generous gift from the Finn lab. PC-12 adherent cells were purchased from ATCC. Nerve Growth Factor 7S was purchased from Life Technologies. 4′,6-diamidino-2-phenylindole (DAPI) was purchased from EMD Biosciences, Inc. Anti-Neuron-specific β-III Tubulin-NL557 was purchased from R&D Systems Inc.

Instrumentation

Multilayer co-extrusion was performed using two-component coextrusion system with 12 multipliers. ATR-FTIR Imaging was conducted on a Digilab FTS 7000 spectrometer, a UMA 600 microscope, and a 32×32 MCT IR Imaging focal plane array (MCT-FPA) image detector with an average spatial area of 176 μm×176 μm in the reflectance mode. Surface analysis of materials was investigated on a PHI Versaprobe 5000 Scanning X-Ray Photoelectron Spectrometer (XPS) with an A1 Kα X-ray source (1486.6 eV photons). Scanning electron microscopy (SEM) was performed using a JEOL SEM under an emission voltage of 20 kV. A high-intensity UV lamp (Bluepoint 4 Ecocure from Honle UV America Inc.) was used for surface modification of the PCL fibers with propargyl benzophenone (Pr-Bz). The molecular weight of the synthesized azido-peptide was measured on a Bruker AUTOFLEX III MALDI-TOF/TOF mass spectrometer using α-Cyano-4-hydroxycinnamic acid (CHCA) as a matrix. Fluorescent images were taken via laser scanning fluorescence confocal microscopy using a Leica TCS SPE Confocal Microscope. Water contact angle (WCA) measurements were tested on a CAM 200 optical contact angle meter (KSV Instruments Ltd). Fluorescent gradient images were collected on a Maestro imaging system from PerkinElmer.

Coextrusion of PCL Fibers

A multilayered film was extruded by a multiplication coextrusion process to fabricate polymeric fibers. PCL (CAPA 6800 pellets, MW=80 kg/mol) was coextruded with poly(ethylene oxide) (PEO) to produce PCL fibers. In order to match the rheology of PCL and PEO melts during the extrusion, two grades of PEO (Dow POLYOX N80 (MW=200 kg/mol) and POLYOX N10 (MW=100 kg/mol)) with a weight ratio of 30:70 were pre-blended using a Haake Rheodrive 5000 twin screw extruder. The viscosities of the obtained PEO blend and PCL melt match at the extrusion temperature, 200° C. Ten vertical multipliers and two horizontal multipliers were used throughout the extrusion line to generate a 256×4 matrix architecture that contains 128×4 PCL domains embedded in PEO. The chill roll speed was 40 rpm and the dimensions of the exit die are 0.5″×0.02″. PEO was removed by stirring in water at room temperature for 24 hours, to yield PCL fiber bundles. The PCL fibers were washed with methanol 3 times and vacuum dried overnight.

Photochemistry

10 mm×30 mm linear gradient images were printed on transparency films (3M PP2410/100) with a commercialized inkjet printer (Epson WF 3540) from transparent to black. To evaluate the gradient photomask, three non-gradient photomasks were produced, corresponding to 3 points (black, 50% black and transparent: 0, 1.5 and 3 cm on the gradient photomask) on the gradient photomask to determine surface immobilization at each point on the fiber (FIG. 11).

Each fiber bundle (3 cm, derived from a single tape) was soaked in a Pr-Bz solution for 5 mins (10 mg/mL in methanol) and was air-dried at room temperature. Samples were placed on a 25 cm×75 cm slide glass and covered with the gradient photomask. The distance between the sample and UV source is 11.5 cm and a 320-390 nm filter was used. The UV intensity varied linearly from 2.7 to 28.6 mW/cm² from black to transparent and the center was 14.6 mW/cm². The unmasked UV source was 33.5 mW/cm². Samples were irradiated for 20 mins After irradiation, samples were washed in methanol overnight to remove excess Pr-Bz, and solvent was evaporated under reduced pressure overnight. The process of photochemistry for non-gradients (PCL-ng-IKVAV) was repeated as described above without the use of a photomask.

Click Chemistry

3 cm of PCL fiber bundles were immersed in a pre-mixed aqueous solution of Azide Fluor 488 (AF₄₈₈) (0.8 mL, 3 mM) or azido-IKVAV (1.5 mL, 1.5 mM), CuSO₄ (10 μL, 50 μM) and THPTA (50 μL, 50 μM). A fresh solution of sodium ascorbate (100 μL, 100 mM) and 500 μL of DI water were added and incubated for 2 hours at room temperature. Gradient modified AF₄₈₈ or IKVAV (PCL-g-AF₄₈₈ or PCL-g-IKVAV) fibers were washed with dimethyl sulfoxide (DMSO) or water, respectively, overnight to remove unreacted dye or peptide. The gradient conjugation of AF₄₈₈ with PCL-Alkyne fibers was confirmed by Maestro fluorescence imaging (green excitation filter used with exposure time 100 ms). Quantification of surface AF₄₈₈ dye was determined by dissolution of the fibers in DCM and quantification by UV-Vis absorbance at 501 nm (FIG. 12). This value was compared to the BET surface area of the fibers to determine surface coverage density (FIG. 13). PCL-g-IKVAV fibers were scanned at 3 spots using ATR-FTIR collecting from 600 to 4000 cm⁻¹ with 128 scans at the resolution 4 cm⁻¹. Spatial distribution of absorbance at 1628 cm⁻¹ was taken using FT-IR Imaging. Atomic chemical composition of PCL-g-IKVAV fibers was analyzed by XPS.

Wettability Test

Measurements were carried out at room temperature with high purity 18.2 MΩ DI water. A single liquid droplet was suspended at the edge of a syringe needle (Matrix Technologies micro-Electrapette 25) and placed on the measured surface. Water droplet images were taken on four different spots across the PCL-g-IKVAV fibers for each sample. The contact angles were measured as right and left angle of the water drop on the surface of the sample and reported as mean angles.

Cell Culture

PC-12 adherent cells (ATCC CRL-1721.1™) were purchased from ATCC and cells were cultured in F-12K Medium containing 15% heat inactivated horse serum, 2.5% fetal bovine serum (FBS), and 1% penicillin. Cells were incubated in 75 cm² cell culture flasks at 37° C., in a 95% air and 5% CO₂ environment. At 80-90% confluency, the cells were detached with PBS/EDTA for 10 minutes at 37° C. The detached cells were collected by centrifugation at 800 g for 5 minutes. 14.5×10⁴ cells were placed on each samples (PCL, PCL-ng-IKVAV and PCL-g-IKVAV) and cultured in the same media with the addition of 50 ng/mL of NGF at 37° C., 5% CO₂ in a humid environment for 5 days. The cell culture medium was refreshed every 2 days.

Immunocytochemical Staining

After 5 days of incubation, all samples were washed with DPBS 3 times and cells were fixed with 4% paraformaldehyde in DPBS for 15 minutes at room temperature. After fixing cells, the samples were washed 3 times with DPBS. Non-specific binding sites were blocked with 10% normal donkey serum, 0.1% Triton® X-100, and 1% BSA in DPBS for 30 minutes at room temperature. Sequentially, cells were incubated with Anti-Neuron-specific β-III Tubulin NL557-conjugated antibody (1:10 dilution) in blocking buffer for 3 hours at room temperature. After 3 hours, cells were washed and nuclei were stained with DAPI (100 μL, 5 μg/ml in PBS) to stain nuclei for 10 minutes and washed 3 times with DPBS.

Synthesis of IKVAV Peptide

The following is a representative coupling cycle for one amino acid monomer—synthesis was carried out in a Torviq 25 mL filtered syringe, specifically designed for peptide synthesis. Peptide synthesis started from 0.5 g of Rink Amide MBHA resin (0.52 mmol/gram) and this resin was pre-swelled in 15 mL of DMF for 10 minutes. The solvent is expelled through the syringe and a solution of 20% 4-methyl piperidine in DMF is added to the resin (10 mL), this is carried out twice for 5 minutes and 20 minutes, respectively. A Kaiser test was performed to confirm Fmoc deprotection. After a positive Kaiser test, the amino acid (0.78 mmol, 3 equiv), HCTU (323 mg, 3 equiv) and DIPEA (272 μL, 6 equiv) were dissolved in a minimal amount of DMF. This solution was added to the resin at room temperature on a rotary shaker and allowed to react for 1 hour. Following the coupling, the resin was again washed with DMF and DCM for 5 mins, 3 times each, and a qualitative Kaiser test was performed to ensure coupling. This synthetic process was repeated until the full peptide sequence was completed. After synthesis of the full peptide (IKVAV), the N-terminus of the peptide was conjugated with 5-Bromovaleric acid—as described above. After washing 3 times with DMF (10 mL) and DCM (10 mL), sodium azide (85 mmol, 5 equiv) was added in DMF and allowed to react overnight. Subsequently, the resin was washed with water to remove excess sodium azide and the resin was dried completely in a lyophilizer. To cleave the peptide, a solution of TFA/H₂O (95/5, v/v) (10 mL) was incubated with the resin for 3 hours at room temperature and the resin was removed by filtration. The cleaved peptide solution was precipitated in cold ether, centrifuged (10,000 rpm for 10 minutes at 4° C.), and decanted to yield a white pellet. The final crude peptide was dried under vacuum to yield a white powder. The crude IKVAV peptides were purified by reverse phase-HPLC (LC-20AD pump, UV-VIS detector SPD-20A, Shimadzu HPLC System). Solution A is 0.05% TFA in water and solution B is 0.05% TFA in acetonitrile. Typical elution gradients were performed using a C18 column (column size: 21.2×250 mm, 7 μm particle size, Agilent ZORBAX 300SB-C18 PrepHT) using a linear gradient from 5 to 95% Solution B over 45 minutes. Collected fractions were pooled and lyophilized to generate a white powder. Azido-IKVAV peptide was confirmed via MALDI-TOF (positive ion mode) using α-cyano-4-hydroxycinnamic acid (CHCA) as a matrix.

Surface Area Measurements

To measure the surface area of PCL fibers, BET measurements were conducted. The samples were degassed with nitrogen gas to remove impurities from the samples at 40° C. overnight. Krypton gas was absorbed on the surface of PCL fibers and 13 data points were collected with the relative pressure ranging from 0.06 to 0.30. The BET plot was generated with a linear region that has a slope of 2.773±0.048 g/cm³ and an intercept of 0.228±0.010 g/cm³ (R²=0.998).

Results

Coextruded Aligned PCL Fiber

In this example, multilayered melt coextrusion of polymeric fibers was chosen for several reasons. The layered coextrusion system uses two polymer components, in this case PCL and PEO, which are both polymers that have been used in several FDA-approved applications. Additionally, both polymers are inexpensive, costing less than $15/kg. Finally, the method is continuous and solvent free, requiring only a water wash to generate PCL fibers, hence eliminating biologically detrimental effects of solvent based processing. During the extrusion process, PCL and poly (ethylene oxide) (PEO) are melt-pumped and layered vertically in the extrusion line. This vertical bilayer was cut in a horizontal plane in the multiplication die and recombined side-by-side to double the number of vertical layers. This process was repeated ten times to increase the number of layers and conversely, decrease the layer thickness. Following vertical multiplication, PEO is layered on the top and bottom of the vertical multilayers. Finally, the multilayered melt undergoes horizontal multiplication, where it is cut in the vertical direction, and the two flow fields are stacked on top of each other to yield PCL fibers embedded in a PEO tape. This procedure was repeated two times, producing a composite extrudate tape containing 512 PCL domains embedded in a PEO tape. The extruded tape is washed in a water bath for ˜24 hours to remove PEO, yielding PCL fibers. Distribution of the size of PCL fibers was examined using scanning electron microscopy (SEM) and was analyzed. The fiber dimensions shows a narrow distribution of size with an average cross-section of 1.49±0.48 μm.

Gradient Photochemistry

In this example, we sought to create a simple method to generate surface-immobilized chemical gradients on aligned extruded fibers. Owing to the ease with which photochemical modification occurs, we aimed to utilize a simple photomasking strategy to modulate the concentration of functionalized benzophenone deposited on the fibers. A 3 cm linear photomask was printed on transparency slides ranging from completely transparent to entirely black (FIG. 12A), allowing us to modulate the UV-intensity for photochemistry by one full order of magnitude. PCL fibers were dip coated in a concentrated solution of propargyl benzophenone (Pr-Bz), allowed to dry, and subjected to UV-irradiation for 20 minutes (FIG. 11).

Upon UV irradiation, Pr-Bz is excited to form a radical species, which can then undergo a radical insertion into the PCL backbone. This resulted in a new covalent bond between Pr-Bz and PCL, leaving surface exposed propargyl groups that could undergo the CuAAC reaction. After alkyne gradient formation, the CuAAC reaction was used to attach Azide Fluor 488 (AF₄₈₈) to the fibers, a green fluorescent dye that allows for simple visualization of the gradient modified surface. The surface gradient was visualized using fluorescence imaging in the green channel, clearly indicating gradient formation (FIG. 12B). The gradient intensity was analyzed by mean fluorescence intensity over the entire fiber bundle to correlate the fluorescent gradient to the linear photomask, showing a strong linear correlation to the mask (FIG. 12C).

Quantification of AF₄₈₈ Gradient

In order to quantify surface coverage following AF₄₈₈ immobilization, dye coverage was quantified using UV-Vis spectroscopy. Dye quantification proves to be much simpler than peptide quantification, and in the past, has correlated well to peptide conjugation. The correlation is likely due to the extreme efficiency of the ligand-accelerated CuAAC reaction. Rather than quantifying the gradient fibers directly, we chose to take individual points within the gradient and understand how UV fluence impacted surface coverage. Three individual photomasks were used to attach Pr-Bz, followed by CuAAC with AF₄₈₈, where photomasks corresponded to (a), (b), and (c) in FIG. 12A (i.e., completely transparent, 50% black, and 100% black) to evaluate surface coverage (FIG. 16). After the CuAAC reaction, fibers were dissolved and dye loading was quantified and compared to BET surface area measurements (18.8 cm²/mg). The quantification of AF₄₈₈ was determined using UV-Vis spectroscopy at 501 nm against a standard curve in dichloromethane following dissolution of PCL-AF₄₈₈ fibers. Fibers with a transparent photomask, corresponding to spot (c) (FIGS. 12A and 16), allowed UV transmittance of 28.6 mW/cm² and led to 0.43 nM/cm² of surface coverage (FIG. 17) on the fiber bundle. An intermediate intensity, corresponding to 50% black in the photomask indicated by spot (b) (FIG. 12A) provided 14.6 mW/cm² of UV intensity. This photomask yielded 0.24 nM/cm² of AF₄₈₈ attachment onto the surface of the fibers (FIG. 17). Finally, the UV intensity of the 100% black photomask, corresponding to spot (a) (FIG. 12A) of the gradient photomask yielded 0.09 nM/cm² of AF₄₈₈ decorated onto the fibers. Gratifyingly, the simple inkjet photomasking technique yielded approximate linear results that allowed us to tune the gradients by nearly an order of magnitude from 0.43 to 0.09 nM/cm² onto the surface.

Gradient Surface Modification of PCL with IKVAV

It is well known that cells can respond to haptotactic gradients, or surface immobilized gradients of biologically active molecules. Such gradients can lead to enhanced cell adhesion and migration relative to non-gradient surfaces. In particular, neuronal cells are especially sensitive to these chemical perturbations and can undergo axonal elongation in the presence of haptotactic gradients of laminin derived peptides, like IKVAV of YIGSR. We sought to employ this phenomenon with our extruded fibers, employing the same linear gradient as described above. An azide-modified IKVAV was synthesized and clicked onto Pr-Bz modified PCL fibers, using the optimized gradient conditions for photochemistry and ligand-accelerated CuAAC (PCL-g-IKVAV). As IKVAV does not provide the ease of quantification of the dye-immobilized molecules, several characterization techniques were carried out to investigate peptide gradient formation. First, PCL-g-IKVAV was analyzed by ATR-FTIR and FTIR imaging. The C═O stretching mode in the amide I region is coupled to the bending of the N—H bond and the stretching of the C—N bond represented between 1620 and 1640 cm⁻¹. The ATR-FTIR spectra at 1628 cm⁻¹ confirmed an increasing amide I band (C═O) across the breadth of the fiber length of PCL-g-IKVAV with detection at the 3 different regions of the sample (FIG. 13B). With increasing transparencies of the photomask, more intense amide I peaks are seen, indicating an increasing gradient of amide bonds, as would be expected for gradient IKVAV formation. FTIR imaging was also employed to visualize the spatial distribution of the C═O amide I band and is shown as a chemical heat map (FIG. 13C) from spots (a) to (c) (scale bar on the right indicates intensity of absorbance, blue to red after normalization). The FTIR imaging result correlates well with the full ATR-FTIR spectra as an increase in the intensity of amide C═O bonds is seen, indicating an increase in concentration of IKVAV.

Water contact angle provides an indication of the hydrophilicity and hydrophobicity on the surface of polymeric substrates. Cell adhesion also depends on the wettability of a scaffold surface, especially since synthetic biocompatible polymers, such as PCL, are hydrophobic which usually limits cell interactions with the scaffold. Incorporating peptides such as IKVAV or YIGSR will likely improve the hydrophilicity of the surface, in addition to interacting with cell surface receptors, further enhancing the cell adhesion properties of the polymeric scaffold. Water contact angles were measured on PCL-g-IKVAV fibers from spot 1 to spot 4 indicating a decreasing water contact angle, and hence an increasing amount of surface-immobilized IKVAV (FIG. 14A). Water contact angle values decreased from 107.8±7.5 to 65.5±3.7° (FIG. 14B), indicating that PCL-g-IKVAV fibers become more hydrophilic as surface density is increased. Furthermore, this follows a linear trend as would be expected from our linear photomask. Therefore, it is likely that both gradient hydrophilicity and receptor specific interactions may play a role in improving cell surface adhesion.

Finally, PCL-g-IKVAV fibers were characterized by X-ray photoelectron spectroscopy (XPS) to determine nitrogen content, a unique atom after the fibers are peptide-modified. XPS wide scan of PCL-g-IKVAV fibers, (FIG. 14C) was performed at 4 distinct spots at increasing gradient densities, and the intensity of the N1s was quantified. An increasing intensity of N1s (relative %) at ˜400 eV was seen, showing successful immobilization of gradiated amounts of IKVAV onto the PCL fibers. The percentages of nitrogen were 3.5, 7.5, 9.8 and 12.0% corresponding to spot 1 to 4 (FIG. 15C) as a result of gradient IKVAV immobilization (see Table 1). This result also shows the linear behavior of the nitrogen to carbon ratio (N/C), confirming linear gradient formation.

TABLE 1 Surface characterization. Relative elemental concentrations of different spots on the PCL-g-IKVAV fibers by XPS and values for water contact angles (right column) Atomic component (%) Sample C (1s) N(1s) O (1s) WCA angle (°) Spot 1 77.9 3.5 18.6 107.8 ± 7.5  Spot 2 71.7 7.5 20.7 89.8 ± 3.9 Spot 3 70.4 9.8 19.8 74.6 ± 8.1 Spot 4 70.1 12.0 18.0 65.5 ± 3.7 In Vitro Characterization of Aligned PCL-g-IKVAV Fibers with PC-12 Cells

To determine the effects of the gradient on neural cells, growth of PC-12 cells was evaluated on three different substrates, unmodified PCL fibers, PCL non-gradient IKVAV (PCL-ng-IKVAV) and PCL-g-IKVAV. PC-12 cells are derived from a pheochromocytoma of the rat adrenal medulla, which can differentiate into neurons and are commonly used as a model cell line in regenerative neural medicine. PC-12 cells were seeded onto the fiber and cultured for 5 days in the presence of nerve growth factor (NGF) to allow for neural differentiation and neurite extension. Following incubation, cells were fixed and stained using a NL557 conjugated anti β-III-tubulin antibody that is specific for neural differentiation (red) and DAPI for the nucleus (blue) (FIG. 15). Confocal microscopy images reveal different neurite extension and cellular density of PC-12 cells on each substrate, as well as in three different regions of each substrate. The density of cells on the unmodified PCL fibers, was significantly lower than that observed on the PCL-ng-IKVAV and PCL-g-IKVAV (FIG. 5). Also the 3 different areas on the unmodified PCL showed little adhesion, differentiation, and directed cell alignment (FIGS. 15A-C). This would be as expected, as PCL provides no biologically active cues and the hydrophobic nature of the surface provides little adhesive capabilities for cells.

The biologically active peptide, IKVAV, is known to bind to β-amyloid precursor protein and has been shown to promote adhesion and neurite outgrowth of PC-12 cells. It was postulated that PCL-ng-IKVAV would lead to improved adhesion but would have less propensity for cell alignment and extension than PCL-g-IKVAV fibers. Confocal images indicate a higher density of cells on non-gradient substrates as higher IKVAV densities are seen throughout the non-gradient fibers (FIGS. 15D-F). Although higher IKVAV densities enhanced cell adhesion on PCL-ng-IKVAV, there is minimal neuronal alignment or differentiation of PC-12 cells with a fixed concentration of IKVAV across the sample. Spreading of neurites was in arbitrary directions in the three different spots as observed via anti β-III-tubulin, and neural differentiation was less significant when compared to gradient modified fibers. When non-gradient substrates are compared to PCL-g-IKVAV, several differences arise. FIG. 15G, corresponds to the lowest concentration of IKVAV on the gradient and shows lower numbers of PC-12 cells, as compared to other regions on the same substrate. The highest cell density was seen on the highest concentrations of IKVAV on the gradient fibers (FIG. 15I). In addition, PC-12 cells had noticeably elongated nuclei and neurites along the axis of the PCL-g-IKVAV fibers. The primary direction was along the gradient of the IKVAV motif, which showed the highest cell extension, differentiation and alignment of neurites, as compared to all other samples (FIG. 15G-I). Regardless of peptide concentration on the gradient surface, neurite extension was seen both in the direction of the fibers and along the axis of the gradient. This result implies that the gradient surface of PCL-g-IKVAV fibers is critical for strong biological activity and that the higher concentrations of IKVAV impacted cell adhesion. The gradient fibers effect differentiation of PC-12 cells and their alignment more so than solely the topographical cues of the aligned fiber direction, although both components are interrelated.

Photochemical gradient modification of a scalable class of melt coextruded fibers allowed for immobilization of azide-modified peptide gradients on the surface of aligned PCL fibers. Melt coextrusion allows for a significantly higher throughput of fiber production when compared to other common fiber processing techniques. This photochemical modification is modular and allows for patterning of surface groups about the fiber. Gradient immobilization of the laminin derived peptide, IKVAV, onto PCL fibers was easily accomplished.

From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. All references, publications, and patents cited in the present application are herein incorporated by reference in their entirety. 

Having described the invention, we claim:
 1. A polymer nanofiber scaffold comprising: a plurality of melt extruded polymer nanofibers, the nanofibers each having a rectangular cross-section defined in part by an encapsulating polymer material that is separated from the nanofibers, the nanofibers including a plurality of click-reactive functional groups extending from portions of outer surfaces of the nanofibers, the functional groups being chemically bound to the nanofibers without degrading polymers chains of the nanofibers, wherein the surface area of the nanofiber is greater than 40.0 cm²/mg of fiber.
 2. The scaffold of claim 1, wherein concentration of functional groups extending from at least one portion is at least about 0.1 nmol/cm².
 3. The scaffold of claim 1, wherein the functional groups are spatially arranged on the nanofibers such that a first portion of the nanofibers has first concentration of functional groups and second portion of the nanofibers has a second concentration of functional groups different than the concentration of first portion.
 4. The scaffold of claim 1, wherein the functional groups are spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the functional groups.
 5. The scaffold of claim 1, wherein the functional groups are spatially arranged on the nanofibers in a concentration gradient.
 6. The scaffold of claim 1, the plurality of click-reactive function groups including a first click reactive functional groups and second click reactive functional groups different than the first click reactive functional groups.
 7. The scaffold of claim 1, wherein the functional groups are chemically bound to the nanofibers with diarylhydroxymethylene linkages that are formed by reaction of a click-reactive functional group substituted diarylketone with the polymer chains of the nanofibers.
 8. The scaffold of claim 4, the click-reactive functional groups comprising at least one of an amine, sulfate, thiol, hydroxyl, azide, alkyne, alkene, carboxyl groups, aldehyde groups, sulfone groups, vinylsulfone groups, isocyanate groups, acid anhydride groups, epoxide groups, aziridine groups, episulfide groups, —CO₂N(COCH₂)₂, —CO₂N(COCH₂)₂, —CO₂H, —CHO, —CHOCH₂, —N═C═O, —SO₂CH═CH₂, —N(COCH)₂, —S—S—(C₅H₄N) and groups of the following structures,

wherein R₁ is hydrogen or C₁ to C₄ alkyl.
 9. The scaffold of claim 1, wherein the nanofibers are formed of a polycaprolactone.
 10. The scaffold of claim 1, wherein the encapsulating polymer material is a water soluble polymer.
 11. A wound dressing comprising: a plurality of melt extruded polymer nanofibers and at least bioactive agent conjugated to the nanofibers, the nanofibers each having a rectangular cross-section defined in part by an encapsulating polymer material that is separated from the nanofibers, the nanofibers including a plurality of click-reactive functional groups of a specific binding pair extending from portions of outer surfaces of the nanofibers, the click-reactive functional groups being chemically bound to the nanofibers without degrading polymers chains of the nanofibers and the at least one bioactive agent being conjugated to a complementary click-reactive group that is appended to the specific binding pair of the click-reactive functional groups of the nanofibers to conjugate the at least one bioactive agent to the nanofibers, wherein the surface area of the nanofiber is greater than 40.0 cm²/mg of fiber.
 12. The dressing of claim 11, wherein concentration of functional groups extending from at least one portion is at least about 0.1 nmol/cm².
 13. The dressing of claim 11, wherein the functional groups are spatially arranged on the nanofibers such that a first portion of the nanofibers has a first concentration of functional groups and a second portion of the nanofibers has a second concentration of functional groups different than the first concentration of the first portion, the functional groups being appended to complementary click-reactive groups that are conjugated to at least one bioactive agent to provide a first concentration of bioactive agents on the first portion and a second concentration of bioactive agents on the second portion.
 14. The dressing of claim 11, wherein the functional groups are spatially arranged on the nanofibers such that different portions of the nanofibers have different concentrations of the functional groups, the functional groups being appended to a complementary click-reactive groups that are conjugated to at least one bioactive agent to provide different concentrations of bioactive agents on the nanofibers.
 15. The dressing of claim 11, wherein the functional groups and bioactive agents are spatially arranged on the nanofibers in a concentration gradient.
 16. The dressing of claim 11, the plurality of click-reactive function groups including a first click reactive functional groups and second click reactive functional groups different than the first click reactive functional groups, the first click reactive functional groups being appended to first bioactive agents and the second click reactive functional groups being appended to bioactive agents different than the first bioactive agents.
 17. The dressing of claim 11, wherein the functional groups are chemically bound to the nanofibers with diarylhydroxymethylene linkages that are formed by reaction of a click-reactive functional group substituted diarylketone with the polymer chains of the nanofibers.
 18. The dressing of claim 17, the click-reactive functional groups comprising at least one of an amine, sulfate, thiol, hydroxyl, azide, alkyne, alkene, carboxyl groups, aldehyde groups, sulfone groups, vinylsulfone groups, isocyanate groups, acid anhydride groups, epoxide groups, aziridine groups, episulfide groups, —CO₂N(COCH₂)₂, —CO₂N(COCH₂)₂, —CO₂H, —CHO, —CHOCH₂, —N═C═O, —SO₂CH═CH₂, —N(COCH)₂, —S—S—(C₅H₄N) and groups of the following structures,

wherein R₁ is hydrogen or C₁ to C₄ alkyl.
 19. The dressing of claim 11, wherein the nanofibers are formed of a polycaprolactone.
 20. The dressing of claim 11, wherein the encapsulating polymer material is a water soluble polymer.
 21. The dressing of claim 11, wherein the bioactive agent promotes at least one of cell adhesion, growth, or proliferation.
 22. The scaffold of claim 1, wherein the melt extruded polymer nanofibers are produced using a multiplication coextrusion process.
 23. The dressing of claim 11, wherein the melt extruded polymer nanofibers are produced using a multiplication coextrusion process. 